A drive force control system or traction control system which prevents a drive wheel from slipping during acceleration of a vehicle and prevents the decline of acceleration performance, suppresses wheel slip for example by decreasing an engine output according to the slip of the drive wheels.
The decrease of engine output may be realized for example by closing a second throttle which is provided in parallel with an intake throttle, by stopping fuel injection to a specific cylinder of the engine, or by retarding the fuel ignition timing of the engine.
Tokkai Hei 4-50440 published by the Japanese Patent Office in 1992 discloses the application of a drive force controller which cuts fuel supply to some cylinders of the engine when drive wheels are slipping, to a vehicle comprising a continuously variable transmission (CVT).
In the CVT, a real speed change ratio is found for example from the ratio of the output shaft rotation speed and the input shaft rotation speed of CVT, and speed change control is performed so that the real speed change ratio coincides with a target speed change ratio which is set according to a running state.
However, since the CVT attempts to decrease the speed change ratio when the engine output torque is decreased due to fuel cut, but the response of the CVT is somewhat slower than that of drive force reduction by fuel cut, a hunting, i.e., fluctuation, of the speed change ratio is apt to occur, and the torque transmitted to the drive wheels may also fluctuate. This torque fluctuation causes vibration of the vehicle body.
For example, during acceleration after vehicle startup, the rotation speed of the drive wheels increases suddenly if the drive wheels slip. The output of the engine is therefore correspondingly reduced by performing fuel cut, the drive wheel speed decreases, and the slip converges. The target speed change ratio of the CVT also varies largely relative to the sudden change of drive wheel speed during this time, but as the response of the CVT is lower than that of the engine output variation due to fuel cut, hunting easily occurs while the drive wheel speed is rapidly varying.
Further, although drive force control due to fuel cut is highly responsive, the control resolution is low, and in an engine with a small number of cylinders, the engine output largely varies due to the fuel cut in one cylinder. Therefore, a variation of drive force easily becomes excessive due to fuel cut operation.